In the oil and gas industry hydrocarbon bearing formations are accessed via drilled bores extending from the surface, with such bores typically lined with metal tubing to provide sealing and support. Many tools have been developed in the art for performing desired operations downhole within the wellbore, such as providing sealing, taking measurements, perforating and the like. One such downhole tool is the bridge plug, which is typically used to seal or isolate a region, such as a lower region, of a wellbore.
The nature of the downhole environment, the depths involved, and the desire to maintain a wellbore in efficient production with minimal interruption call for tools which are extremely robust and reliable. That is, operators must have confidence that any tooling used within a wellbore will reliably perform its desired function and will not fail while downhole, which could require remedial action such as rescuing the tool, drilling out the tool or even abandoning the wellbore, all at significant delay to production with associated financial consequences.
Many downhole tools may require to become anchored within the wellbore. Known designs, such as might be used in bridge plugs, utilise slips which are radially extended from the body of a tool to engage the inner surface of the wellbore, thus locking the tool in a fixed axial location within the wellbore. Some known slips may be mounted on a cone which is axially displaced to radially extend the slips. In prior art arrangements a slip may only be supported at the point of contact with the cone such that any forces established when the slip engages a bore wall will only be reacted through the point of engagement with the cone. This may focus significant levels of stress in both the cone and the slip which may be undesirable and establish a potential point of failure.
Also, it is desirable in many cases for downhole tools to be retrievable, requiring the ability for anchor slips to be retracted, and maintained in a retracted configuration while the tool may be retrieved from the wellbore without hindrance. Such retraction may be provided by an interengaging profile arrangement, such as a dovetail arrangement, between the slips and an actuating cone. However, such an interengaging profile may be subject to failure, such as by blockage, plastic or complete mechanical failure or the like. Further, it may also be known in the art to utilise spring return systems. However, the structural arrangements of known slip designs and the minimal available space may only permit the use of springs with relatively low spring force, which may be insufficient to achieve retraction of a slip.
Many downhole tools such as those which incorporate anchor slips require to be actuated by application of a setting force, for example an axial setting force to drive an actuating cone. Such a setting force may be provided by a separate setting tool. However, it may be undesirable to continuously apply a setting force via a setting tool to maintain a set of slips, for example, in an extended configuration. In such cases it is known to provide a locking or ratchet system which permits a setting configuration to be locked-in permitting the setting force to be removed. Many forms of ratchet system exist, and normally include a ratchet component having ratchet teeth which engage corresponding teeth in a ratchet body. As most downhole tools are generally cylindrical it is common to utilise ratchet components which are, in essence, curved segments and often these segments have minimal dimensions due to space restrictions. It is often the case, therefore, that the forces applied through the ratchet teeth when engaged are only applied over minimal areas, which may establish significant stresses through the components of the ratchet system and creating a possible failure point. Furthermore, due to the potential hold-forces involved, it may be difficult to apply the necessary release force, without damaging the ratchet components.